European Union foreign policy chief Federica Mogherini is appealing on the Syrian government and Russia to do their utmost to spare civilians as they prepare to capture the last rebel holdouts in the besieged city of Aleppo.

Mogherini said in a statement on Wednesday that "the priority now, in these hours, is to protect civilians, guarantee them safe and monitored transit to a place of safety."

She added that "this is particularly the responsibility of the Syrian government and its allies." Mogherini also warned that "those who perpetrate war crimes will be held accountable."

Media reports said that buses, which were meant to evacuate rebels and tens of thousands of civilians from Syria's eastern Aleppo, have left the crossing point on the edge of the opposition enclave and returned to their depots.

The buses' departure could signal a major delay in the evacuations and even the collapse of the cease-fire deal.

The Lebanese channel al-Manar TV has broadcasted footage showing Syrian government's green-colored buses leaving the evacuation point without any passengers on Wednesday.

Al-Manar is the media arm of the Lebanese militant Shiite group Hezbollah, which is fighting alongside Syrian President Bashar Assad's government forces in Syria.

Shelling and rocket fire also resumed Wednesday at the edges of the opposition's one-square-mile enclave in Aleppo.